Adaptive colouration in wildebeests, part 8: by night

Studying the colouration of ungulates by means of the centrally focussing retina of a diurnal, non-colourblind primate is a mixed blessing. On one hand, I can peruse the many photos in iNaturalist, which would be beyond the eyes of non-primate mammals designed to scan for motion rather than to home in on static detail. On the other hand, it sets me up to overplay reddish and other hues (which neither ungulates nor carnivores are likely to notice even in bright light, see https://en.wikipedia.org/wiki/Equine_vision) and to underplay the views that arguably matter most: those in the dark.

The adaptive value of colouration in ungulates often seems discouragingly inscrutable, but we should imagine exposure to predation at night. Running in the dark is hazardous because of unseen obstacles. The crux shifts from fleeing to preventing detection in the first place, perhaps even for the most cursorial of 'plains game'. So notwithstanding that both ungulates and carnivores see much more in dim light than we do, the rules of adaptive colouration may bend at night.

Take the following puzzles, for example.

Why do wildebeests combine disruptive brindling with a lack of countershading (see taurinus eastern form https://www.inaturalist.org/observations/58666556 and https://www.inaturalist.org/observations/70162483)? By day, this combination is conspicuous because the dark-shaded ventral surface of the torso makes the brindling ineffective (see https://en.wikipedia.org/wiki/Countershading). But by starlight there is no shading, and the disruptive brindling (https://en.wikipedia.org/wiki/Disruptive_coloration) could make all the difference as the figure stands still to blend into its surroundings (https://www.flickr.com/photos/16898659@N07/4178414593/).

Why, in wildebeests as well as e.g. large bovines, is the general tendency in conspicuous species for dark rather than pale overall colouration? Either conspicuously dark or conspicuously pale would suit gregarious ungulates by day, but by night paleness becomes a liability. This can also apply at a smaller scale, to the colouration of the face. Facial flags in wildebeests consist of pale cheeks offset by a dark front-of-face (see albojubatus https://www.kimballstock.com/preview.asp?dc=a&image=AFW%2003%20MH0029%2001&pb=&cs=&pbx=&itemw=4&itemf=0001&itemstep=1&itemx=3). This is conspicuous even at a distance by day, when the profiled head is moved. However it would be hidden from scanning predators at night, when the wildebeest turns its dark face towards the approaching cause of the alarm while keeping the head still.

Why do infants and young juveniles of several forms of wildebeest have such pale feet (e.g. see albojubatus https://www.dreamstime.com/stock-photo-newborn-wildebeest-amboseli-park-kenya-mother-baby-image76472324) that they may possibly qualify for a pedal flag? By night this may be just visible enough to aid group-cohesion after alarm without being spotted by the predators causing the alarm.

Posted on 11 July, 2021 06:01 by milewski